PCSI2023 Session PCSI-TuM1: Electronic and Structural Properties of Films and Surfaces

Strain solitons are topological defects which have been observed in van der Waals materials [1]. Strain solitons are generated as a result of thin film strain and form as ripples in the films, separating commensurate and incommensurate regions. The solitons are analogous to misfit dislocations and cause local relaxation of the crystal. Solitons have been of interest due to their potential for generating novel spatially confined states within two-dimensional materials [2].

In this talk, we report the formation of strain solitons in epitaxial (111) bismuth thin films grown by molecular beam epitaxy on InSb(111)B substrates. The 0.9% lattice mismatch between bulk bismuth and the InSb(111)B substrates results in strain relaxation via strain soliton formation and in-plane edge dislocation formation. Edge dislocations are observed to induce large angle bending of the solitons. The solitons appear to form at the interface and propagate throughout the film. The strain state of the bismuth thin films is characterized by in-situ low temperature scanning tunneling microscopy and ex-situ X-ray diffraction. The effect of the solitons on local strain and lattice rotation is mapped from the STM images using the Lawler-Fujita algorithm [3]. Partial relaxation of bismuth is observed to begin at 2 bilayers, and the thin films become fully relaxed by 30 bilayer thick. The evolution of the soliton network and their strain fields as a function of thickness is characterized. Our results indicate that bismuth primarily relaxes by strain soliton formation and enables future investigation of novel states hosted in the bismuth solitons.

References

[1] PNAS 110(28), 11256-11260 (2013).

[2] PRL 121, 266401 (2018).

The growth and structure of the cubic manganese nitrides, namely θ- MnN, η- Mn₃N_2, and ε- Mn₄N, has already been investigated intensively on MgO (001) substrates [1,2]. However, the hexagonal ζ- Mn₂N has remained unexplored. Thin films of hexagonal ζ- Mn₂N were grown successfully on MgO (001) using molecular beam epitaxy (MBE) and used as a substrate to grow a very thin layer of bismuth.

In this presentation, the epitaxial growth of ζ- Mn₂N and multilayer of bismuth will be discussed. The sample growth process was monitored by in-situ reflection high energy electron diffraction. RHEED shows a streaky pattern indicating the smooth sample surface throughout the sample growth. During the Mn₂N growth, 1/4^th order fractional streaks are observed along [100]_MgO + R30° direction. These fractional streaks indicate the 4× reconstruction on the Mn₂N surface. The presence of a 4× reconstructed surface is also verified by room temperature scanning tunneling microscopy. Moreover, the RT-STM images show the atomically flat terraces, steps, and the atomic resolution of the hexagonal array of the Mn₂N surface. Various in-situ and ex-situ measurements are performed to calculate the in-plane and out-of-plane lattice constants. The calculated lattice parameters are in good agreement with the theoretically reported values of ζ- Mn₂N (a=b= 2.83 Å, c= 4.54 Å) [3]. Furthermore, the surface chemistry of the samples was determined by in-situ Auger electron spectroscopy at different locations on the sample surface. The stoichiometric ratio of Mn: N on the film is nearly 2:1 which is consistent with ζ- Mn₂N. After successfully growing ζ- Mn₂N, a very thin film of Bi is deposited, and the surface is studied by RT-STM. Interestingly, the measurements show multiple steps with a height of ~1.6 Å which is not matching the step height of bulk Bi (111) (3.94 Å) [4], but it is consistent with the step height of bismuthene grown on Ag (111) as reported by Sun et al. (2022) [5]. Although the step heights suggest multilayer bismuthene, atomic resolution images show a rectangular lattice.

[1] H. Yang, H. Al-Brithen, E. Trifan, D. C. Ingram, and A. R.Smith, J. Appl. Phys. 91, 3 (2002).

[2] A. Foley, J. Corbett, K. Alam, D. C. Ingram, and A. R. Smith, J. Crystal growth446, 60 (2016).

[3] M. Aoki, H. Yamane, M. Shimada, and T. Kajiwara, Material Research Bulletin39, 827 (2004).

[4] H.- H. Sun, M.- X. Wang, F. Zhu, H. Ma, Z.-A. Xu, Q. Liao, Y. Lu, Y.-Y. Li, C. Liu, D. Qian, D. Guan, and J.-F. Jia, Nano Lett.17, 3035-3039 (2017).

[5] S. Sun, J. Yang, S. Duan, J. Chen, and W. Chen, ACS Nano16, 1463-1443 (2022).

Group IV GeSn alloys are extensively studied for efficient Si-based photonics. The interest is elevated recently due to the demonstration of direct bandgap emission in Ge [1]. Specifically, a combination of strain and n-type doping reduces the small separation (~140 meV) between the indirect (L) and direct (Г) valleys in Ge [2] and raises the Fermi level for sufficient electron population in the Г valley. Nonetheless, the high level of strain (2%) and doping (10¹⁹ cm^-3) only allows a tunable Ge bandgap between 1.3 and 1.6 μm. Alternatively, alloying Ge with the negative bandgap α-Sn results in a fundamental direct bandgap GeSn material with better tunability.The theoretical and experimental estimations indicate an indirect to direct bandgap transition for Sn compositions above 6%.

Achieving Sn-rich GeSn alloys is difficult. This relates to (i) the low equilibrium solid solubility of Sn in Ge (<1%) and (ii) the large lattice mismatch between Ge and α-Sn (15%). However, Sn-rich GeSn alloys have been demonstrated by non-equilibrium growth techniques. The thermal stability of such structures is an issue. We study the structural and optical properties of CVD grown GeSn films in detail. The strain relaxation and Sn segregation is reported for thermal treatment at 300 °C. We discuss on the mechanism of Sn segregation trough the interaction with dislocation.

Group IV alloys of Ge and Sn (Ge_1-xSn_x) are extensively studied for various electronic and optoelectronic applications on a Si platform. Alloying GeSn with α-Sn concentrations as low as 6% [1] allows for an indirect to direct optical transition compared to Ge. Higher Sn content makes possible mid and even long-range infrared optical emission and detection [2,3]. At the same time, due to the low solid solubility of Sn in Ge (~1%) as well as the large lattice mismatch of α-Sn with Ge (~14%), the realization of Sn-rich Ge_1-xSn_x structures is challenging.

We will discuss the growth and investigation of crystalline Ge_1-xSn_x heteroepitaxial layers with Sn percentage higher than 14% on InAs (001) substrates. Utilizing the small lattice mismatch between Ge_1-xSn_x layers and the InAs (001) substrate, we achieved epitaxial growth of Ge_1-xSn_x with a high Sn percentage (>30%) by applying a high Sn gradient during growth. The strain induced in the GeSn layers was investigated and the effect of gradually increasing the Sn content of the GeSn lattice significantly affected the composition and morphology of the GeSn layers. Supporting data (Figs 1-3) on the structural properties of Ge_1-xSn_xalloys using X-ray diffraction, SIMS analysis, XTEM and Raman spectra will be discussed and are shown below.

We report our recent results on the development of ultra-thin black Phosphorus (bP)/GaAs junction photodiodes for infrared (IR) detection. The devices comprise an epitaxially grown n-type GaAs nanomembrane and a p-type bP flake exfoliated from commercially available bulk crystals. bP is a promising material for the realization of visible-to-mid-IR detectors as its direct bandgap can be tuned from 0.3 eV to 2 eV by varying its thickness at the nanoscale [1]. The GaAs nanomembrane is multifunctional in that it serves as the n-side of the IR junction photodiode, and it is expected to shield the device from radio-frequency (RF) waves as it has been heavily doped. The fabricated devices have a strong potential to implement RF-waves hardened electronic sensing of IR radiation, a capability of tremendous interest for military and commercial applications. An additional value of the GaAs/bP ultra-thin photodiodes is that they can conform to non-planar surfaces such as aircraft windows and domes of IR cameras, and they can be readily integrated into soldiers’ uniforms.

We have established a process to integrate large-area bP flakes onto 220 nm-thick GaAs nanomembranes and used conventional top-down photolithography to fabricate bP-GaAs p-n diodes with areas up to 1.27 x 10^-4 cm². Measured dark-current-voltage characteristics showed a turn-on voltage of 1.4 V with a maximum forward current density of ~ 90 A/cm² at 2 V and a dark current density of ~ 2 A/cm² at -2 V. We correlate the materials and devices parameters extracted from the current-voltage characteristics with electronic band-structure diagrams and the structure of surfaces and interfaces probed by electron microscopy and x-ray photoelectron spectroscopy. In this talk, we will show the geometry of the bP/GaAs photodiode and the physical-chemical structure of surfaces and interfaces at the nanoscale determine the dominant transport mechanisms and the performance parameters of the device.

[1] X. Chen, X. Lu, B. Deng, O. Sinai, Y. Shao, C. Li, S. Yuan, V. Tran, K. Wantanabe, T. Taniguchi, D. Naveh, L. Yang, and F. Xia, Nat. Commun. 8, 1 (2017)

It is of significant importance but a challenging task to probe how local chemical environments affect single-molecule behaviors with angstrom scale resolution. Tip-Enhanced Raman Spectroscopy (TERS) affords the spatial resolution of traditional Scanning Tunneling Microscopy (STM) while collecting the chemical information provided by Raman spectroscopy. By using a plasmonically-active probe tip, the Raman signal at the tip-sample junction is incredibly enhanced, allowing for single-molecule probing. This method, further aided by the benefits of ultrahigh vacuum, is uniquely capable of controlling localized plasmons via an atomistic approach. We are able to obtain (1) single-molecule chemical identification;¹ (2) adsorbate-substrate interactions of individual molecules on the surface;² (3) local strain effects in 2D heterostructures;³ (4) atomic-level insights into surface oxidation reactions.⁴ By investigating substrate structures, molecular superstructures, 2D material lattices, and adsorption geometries obtained from vibrational modes, we extract novel surface-chemistry information at an unprecedented spatial (< 1 nm) and energy (< 10 wavenumber) resolution. Another important application of localized surface plasmons is to achieve site-selective chemical reactions at sub-molecular scale. Recently, we selectively and precisely activated multiple chemically equivalent reactive sites one by one by scanning probe microscopy tip-controlled plasmonic resonance.⁵ Our method can interrogate the mechanisms of forming and breaking chemical bonds at the angstrom scale in various chemical environments, which is critical in designing new atom- and energy-efficient materials and molecular assemblies with tailored chemical properties.

Reference:

1. S. Mahapatra, Y. Ning, J. F. Schultz, L. Li, J. -L. Zhang, N. Jiang, Nano Letters,19, 3267-3272 (2019).

2. J. F. Schultz, L. Li, S. Mahapatra, C. Shaw, X. Zhang, N. Jiang, The Journal of Physical Chemistry C, 124, 2420-2426 (2020).

3. L. Li, J. F. Schultz, S. Mahapatra, X. Zhang. X. Liu, C. Shaw, M. Hersam, N. Jiang, Journal of the American Chemical Society, 143, 38, 15624-15634 (2021).

4. L. Li, J. F. Schultz, S. Mahapatra, Z. Lu, X.Zhang, and N. Jiang, Nature Communications, 13, 1796 (1-9) (2022).

5. S. Mahapatra, J. F. Schultz, L. Li, X. Zhang, N. Jiang, Journal of the American Chemical Society, 144, 5, 2051-2055 (2022).

Author for correspondence: njiang@uic.edu

Solar-powered water splitting using nanoparticle photocatalyst suspensions is a promising route to economical, clean hydrogen production. In the Z-scheme approach, hydrogen and oxygen-evolving photocatalysts, such as SrTiO₃:Rh and BiVO₄, are coupled with a redox mediator to improve light absorption compared to single-photocatalyst systems. A key step in the water-splitting process is the separation and transport of photo-excited electrons and holes to the photocatalyst surface. Here we characterize charge transport in individual SrTiO₃:Rh and BiVO₄ nanoparticles using a nanoprobe within a scanning electron microscope, and directly map internal electric fields with electron-beam induced current. Charge transport in SrTiO₃:Rh particles is limited by bulk defect states within the nanoparticle, in contrast to nearly Ohmic conduction in BiVO₄ nanoparticles. SrTiO₃:Rh particles contain insignificant built-in E fields, while BiVO₄ nanoparticles contain built-in E field between different facets of the nanoparticle which can efficiently separate e-h pairs. Inefficient charge transport and lack of built-in electric field explain why the H₂-evolving SrTiO₃:Rh nanoparticles are the limiting component within this Z-scheme system.

The current copper interconnect technology beyond the 7nm node is facing a crucial challenge of rapidly increasing metal line resistance as the downscaling of device dimensions [1]. In a narrow metal linewidth, the electrical resistivity of Cu increases rapidly due to its relatively high electron mean free path increasing the surface electron scattering. Recent studies have disclosed that binary intermetallic compounds such as NiAl have a high potential to replace Cu due to their low resistivity and high reliability at small dimensions [2]. The critical drawback of NiAl, however, is its rapid increase of resistance upon post-annealing due to the formation of surface oxide and subsequent composition shift of the compound [3].

In this talk, we propose the employment of an aluminum oxide capping layerwith thickness varied from 1-5 nm to improve the thermal stability of ultrathin NiAl films. We found that employing a capping layer effectively prevented the Al out-diffusion from NiAl to aluminum oxide thus the compositional change was no occurred during thermal annealing. Thickness-dependent resistivity of NiAl films was measured, and the NiAl film with 1-nm-thick capping layer shows remarkably low resistivity (49.7mW∙cm) for 3.2 nm NiAl film after annealing at 450°C.

We explore the frequency-dependent conductivity, σ, of granular metals (GMs). Granular metal comprise nanoscale metal islands embedded in a dielectric matrix. We target the metal volume fraction, φ, so that the few-nm diameter metal islands are separated by a ~1 nm dielectric barrier. These metal/insulator composites can be modelled as a complex resistor-capacitor network with complementary tunneling and capacitive conduction paths [1]. At low frequencies, ideal GMs are highly insulating with thermally-assisted tunneling between islands occurring at high E-fields. At high frequencies, capacitive transport dominates; conductivity increases orders of magnitude.

We compare M-SiN_x and M-YSZ (M = Mo or Co; YSZ = yttria-stabilized-zirconia) GM thin films grown via rf co-sputtering on sapphire. Frequency-dependent conductivity is evaluated using impedance spectroscopy at ambient and cryogenic temperatures. Temperature and field-dependent DC conductivity provide insight into tunneling mechanisms. Granular metal structure (Fig. 1a, b) and composition are determined by scanning transmission electron microscopy (STEM) and x-ray photoemission spectroscopy (XPS).

Through careful selection of the metal-insulator system and synthesis optimization, we prepared GM films having σ_MHz/σ_DC >10⁵ (Fig. 1c). This σ_MHz/σ_DC ratio was achieved for Mo-SiN_x using deposition conditions that significantly reduced the vacancies present in the sputtered insulator. Analysis of metal/insulator interfaces via XPS showed metal-oxide formation in M-YSZ and metal-silicide formation in M-SiN_x [2]. Control of deposition conditions can minimize these defects, allowing σ_MHz/σ_DC optimization for high-pass filters.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories (SNL). SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2022-12964 A.